Table of Contents

What is Gear Train

Gears are used for power transmission from one shaft to another. Sometimes, two or more gears are made to mesh with each other to transmit power from one shaft to another. Such a combination is called a gear train or train of toothed wheels.

Gear train Types depends on the following factors :

The relative position of the axes of shafts.
Velocity ratio required.

Gear train and its types

Following are the different types of gear trains, depending upon the arrangement of wheels:
1. Simple gear train,
2. Compound gear train,
3. Reverted gear train, and
4. Epicyclic gear train.

In the first three types of gear trains, the axes of the shafts over which the gears are mounted are fixed relative to each other. But in case of epicyclic gear trains, the axes of the shafts on which the gears are mounted may move relative to a fixed axis.

Simple Gear Train

When there is only one gear on each shaft, it is known as a simple gear train.
When the distance between the two shafts is small, the two gears 1 and 2 are made to mesh with each other to transmit motion from one shaft to the other, as shown in Fig. 1. Since the gear 1 drives the gear 2, therefore gear 1 is called the driver and the gear 2 is called the driven or follower.
It may be noted that the motion of the driven gear is opposite to the motion of driving gear.

Sometimes, the distance between the two gears is large. The motion from one gear to another, in such a case, maybe transmitted by either of the following two methods :
1. By providing the large-sized gear, or
2. By providing one or more intermediate gears.
The idle gears ( Gear number 2 in fig . b ) are used for the following two purposes:
1. To connect gears where a large center distance is required, and
2. To obtain the desired direction of motion of the driven gear (i.e. clockwise or anticlockwise)
Let N1 = Speed of gear 1(or driver) in r.p.m.,
N2 = Speed of gear 2 (or driven or follower) in r.p.m.,
T1 = Number of teeth on gear 1, and
T2 = Number of teeth on gear 2.

Since the speed ratio (or velocity ratio) of the gear train is the ratio of the speed of the driver to the speed of the driven or follower and ratio of speeds of any pair of gears in mesh is the inverse of their number of teeth, therefore

Speed Ratio = ( N1 / N2 = T2/T1 )
Speed Ratio = ( Speed of Driver / Speed of Driven ) = ( No. of teeth of Driven / No. of teeth of Driver)

Compound Gear Train:

When there is more than one gear on a shaft, as shown in Fig. , it is called a compound train of gear.
We have seen in the previous section that the idle gears, in a simple train of gears do not affect the speed ratio of the system. But these gears are useful in bridging over the space between the driver and the driven. But whenever the distance between the driver and the driven or follower has to be bridged over by intermediate gears and at the same time a great ( or much less ) speed ratio is required, then the advantage of intermediate gears is intensified by providing compound gears on intermediate shafts.
In this case, each intermediate shaft has two gears rigidly fixed to it so that they may have the same speed. One of these two gears meshes with the driver and the other with the driven or follower attached to the next shaft as shown in Fig.

The advantage of a compound train over a simple gear train is that a much larger speed reduction from the first shaft to the last shaft can be obtained with small gears. If a simple gear train is used to give a large speed reduction, the last gear has to be very large. Usually for a speed reduction in excess of 7 to 1, a simple train is not used and a compound train or worm gearing is employed.

Speed Ratio = ( Speed of First Driver / Speed Of Last Driven ) = ( Product of the number of teeth on the driven/ Product of the number of teeth on the driver)

Reverted gear train

Reverted Gear Train: When the axes of the first gear (i.e. first driver) and the last gear (i.e. last driven or follower) are co-axial, then the gear train is known as reverted gear train as shown in Fig.
A Reverted gear train is extremely just like a compound gear train. They’re both used when there’s a desire of huge changes in speed or power output but the space for accommodating several gears is restricted.
A reverted gear train is also a compound gear train, having the input and output shafts on the same axis (Co-axial). If these two gears are mounted on the same shaft, one of them must be loosely mounted. And also the distance between the centers of the two gears in each pair must be the same.
The gear transmission ratio is calculated similarly like a compound gear train.

Epicyclic Gear Train:

Epicyclic gearing also called as planetary gearing. It is a gear system that consists of one or more outer gear (planet gear) rotating about a central (sun gear). The planet gear is mounted on a moveable arm (carrier) which itself may rotate relative to the sun gear. Epicyclic gearing systems may also incorporate the use of an outer ring gear or annulus, which meshes with the planet gears.

The figure shows an example of epicyclic gearing. It is used to increase output speed. The planet gear carrier is driven by an input torque. The sun gear provides the output torque, while the ring gear is fixed.

The epicyclic gear trains are useful for transmitting high-velocity ratios with gears of moderate size in a comparatively lesser space. The epicyclic gear trains are used in the back gear of lathe, differential gears of the automobiles, hoists, pulley blocks, wristwatches, etc